patch 9.0.1686: undotree() only works for the current buffer

Problem:    undotree() only works for the current buffer
Solution:   Add an optional "buffer number" parameter to undotree().  If
            omitted, use the current buffer for backwards compatibility.

closes: #4001
closes: #12292

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: zeertzjq <zeertzjq@outlook.com>
Co-authored-by: Devin J. Pohly <djpohly@gmail.com>
diff --git a/runtime/doc/usr_41.txt b/runtime/doc/usr_41.txt
index 2d6792a..2112fbb 100644
--- a/runtime/doc/usr_41.txt
+++ b/runtime/doc/usr_41.txt
@@ -1373,7 +1373,7 @@
 	libcallnr()		idem, returning a number
 
 	undofile()		get the name of the undo file
-	undotree()		return the state of the undo tree
+	undotree()		return the state of the undo tree for a buffer
 
 	shiftwidth()		effective value of 'shiftwidth'